1. Field of the Invention
Embodiments of the disclosure relate to a liquid crystal display and a method of driving the same for solving a distortion phenomenon of the image quality resulting from external light.
2. Discussion of the Related Art
Liquid crystal displays generally display an image by controlling a light transmittance of a liquid crystal layer using an electric field applied to the liquid crystal layer in response to a video signal. Because the liquid crystal displays are small-sized thin profile flat panel displays with low power consumption, the liquid crystal displays have been used in personal computers such as notebook PCs, office automation equipment, audio/video equipment, and the like. In particular, because active matrix type liquid crystal displays whose each liquid crystal cell includes a switching element can actively control the switching elements, the active matrix type liquid crystal displays have an advantage in displaying a moving picture.
A thin film transistor (TFT) has been mainly used as the switching element of the active matrix type liquid crystal displays.
As shown in FIG. 1, an active matrix type liquid crystal display charges a liquid crystal cell Clc to a data voltage by converting digital video data into an analog data voltage based on a gamma reference voltage and simultaneously performing a supply of the analog data voltage to a data line DL and a supply of a scan pulse to a gate line GL. For the above-described operation, a gate electrode of a TFT used as a switching element is connected to the gate line GL, a source electrode of the TFT is connected to the data line DL, and a drain electrode of the TFT is connected to a pixel electrode of the liquid crystal cell Clc and an electrode at one side of a storage capacitor Cst. A common voltage Vcom is supplied to a common electrode of the liquid crystal cell Clc. When the TFT is turned on, the storage capacitor Cst is charged to the data voltage applied through the data line DL to keep a voltage of the liquid crystal cell Clc constant. When the scan pulse is applied to the gate line GL, the TFT is turned on. Hence, a channel is formed between the source electrode and the drain electrode of the TFT, and a voltage on the data line DL is supplied to the pixel electrode of the liquid crystal cell Clc. An arrangement state of liquid crystal molecules of the liquid crystal cell Clc changes because of an electric field between the pixel electrode and the common electrode, and thus incident light is modulated.
The liquid crystal display generally displays an image according to a previously determined gamma curve of 1.8 to 2.2 gamma irrespective of a watching environment (for example, a illuminance of external light). However, the image quality a user perceives may be easily distorted by changes in the watching environment. A distortion phenomenon of the image quality is described with reference to FIGS. 2 to 4. FIG. 2 illustrates an image in a living room environment of a middle brightness, FIG. 3 illustrates an image in a relatively brighter living room environment than the middle brightness, and FIG. 4 illustrates an image in a relatively darker living room environment than the middle brightness. In FIGS. 2 to 4, a gamma curve means a curve obtained by connecting output luminances respectively corresponding to input gray levels, and the brightness means a relative brightness of an image perceived when the user perceives the output luminance being an absolute concept.
To prevent the distortion phenomenon of the image quality, as shown in FIG. 2, the relative brightness of the image has to be kept at an original brightness level of the image irrespective of changes in the watching environment and must have a good linearity in all of gray level periods. However, as shown in FIG. 3, a relative brightness of an image in the brighter living room environment is less than an original brightness level of the image and does not have a good linearity in a low gray level region “A” because of a sensitivity reduction resulting from a reduction of an iris stop. Hence, it is difficult for the user to perceive the image in the low gray level region “A” of the brighter living room environment. Further, as shown in FIG. 4, a relative brightness of an image in the darker living room environment is greater than an original brightness level of the image and does not have a good linearity in a low gray level region “A” and a high gray level region “B” because of a sensitivity improvement resulting from an increase of an iris stop. Hence, in the darker living room environment, a contour occurs between gray levels in an image of the low gray level region “A”, and a glare phenomenon occurs in an image of the high gray level region “B”.
As described above, the distortion phenomenon of the image quality in the specific gray level regions as shown in FIGS. 3 and 4 is caused by the fact that the image is displayed according to the previously determined gamma curve irrespective of changes in the watching environment. In the related art, a method of modulating a gamma curve in a specific gray level range was proposed so as to improve visibility at the specific gray level range. However, the method does not consider the fact that a relative brightness of an image a user perceives must be kept at an original brightness level of the image irrespective of changes in watching environment and must have a good linearity in all of gray level periods. Therefore, the related art has a limit in uniformly keeping the relative brightness of the image the user perceives at an original brightness level of the image irrespective of changes in the watching environment.